With composite materials, it is possible to utilize the exceptional mechanical properties of certain materials, the making of which is known, not in bulk form but only as filaments. Matrices based on organic polymers are then used for binding the filaments together. But, if the intention is to obtain high performance composite materials, it is indispensable that the matrices themselves have good mechanical properties.
Now, generally, the matrices obtained from so-called “thermosetting” resins (as opposed to so-called “thermoplastic” resins) such as epoxide and vinyl ester resins, have poor elongation at break and impact resistance.
It has been known for a long time that the toughness (or “fracture toughness”) of a material, i.e. its capability of opposing sudden propagation of cracks, may be improved by incorporating particles of small dimensions therein. Indeed, when a crack is formed in the material, the propagation of this crack is slowed down by the particles which act as obstacles. Plastic deformation mechanisms and notably cavitation and shear band phenomena then ensure strengthening of the material.
The idea generally considered for overcoming the low toughness of matrices obtained from thermosetting resins is therefore to incorporate into these resins a thermoplastic polymer, the role of which is to concentrate the stresses and to control the released energy upon an impact or cracking of the matrices.
Typically, this incorporation is carried out in two steps: a first step which consists of dissolving the thermoplastic polymer in the thermosetting resin until a homogeneous mixture is obtained, and a second step which consists of curing the thereby obtained mixture by hot polymerization/crosslinking of the resin. De-mixing then occurs, i.e. separation of phases, which leads to the formation of thermoplastic nodules within the cured resin.
In the case of matrices based on hot polymerizable/crosslinkable epoxide resins, it is common to use as a thermoplastic polymer, a polymer from the family of polyethersulfones (or PES) which are amorphous polymers having a high glassy transition temperature, typically about 200° C. An example of such a use is described in the U.S. Pat. No. 5,434,226 (reference [1]).
On the other hand, it is known that polyethersulfones cannot be used for increasing the toughness of matrices based on vinyl ester resins, attempts for incorporating a polyethersulfone into a vinyl ester resin having actually ended in failure because of the non-miscibility of both of these types of compound (Wang S. et al., Journal of Polymer Science Part B: Polymer Physics, 2000, 38, 2409-2421 (reference [2]); Liang, G. et al., Journal of Materials Science, 2005, 40, 2089-2091 (reference [3]).
Consequently, in the European patent application published under no. 1 473 325 (reference [4]), increasing the toughness of a matrix based on a vinyl ester resin was proposed by mixing this resin with a copolymer of arylene ether diphenylsulfone and of arylene ether triphenylphosphine (or BPA-Px) and then, after de-mixing the resulting mixture, by cold polymerizing/crosslinking the resin, i.e. in the absence of any heating.
It is found that this solution is highly effective since it leads to gains in toughness at least equal to 60% and which may attain 88% and this without altering the other properties of the matrix which are inherent to the use of a vinyl ester resin.
However, it is not totally satisfactory insofar that it cannot be applied to vinyl ester resins capable of polymerizing/crosslinking at temperatures below 70° C. on the one hand and the BPA-Px copolymers are relatively expensive products on the other hand.
The Inventors therefore set their goal to find another solution to the problem of the low toughness which matrices obtained from vinyl ester resins have, which itself is applicable both to vinyl ester resins, for which polymerization/crosslinking can only be carried out under hot conditions and to cold polymerizable/crosslinkable vinyl ester resins, and which is less costly than the one recommended in the aforementioned reference [4].
Now, it is found that within the scope of their work, the Inventors noticed that not only is it possible to make a vinyl ester resin and a thermoplastic polymer of the polyethersulfone type miscible by using a reactive diluent in which they are both soluble but also further the joint use of such a polymer and of such a diluent is expressed by gains in toughness which are still higher than those obtained with a BPA-Px copolymer.
And it is on these observations that the present invention is based.